Conventionally, for example, as disclosed in Japanese Laid-Open Patent Application 4-121260, a brake force control apparatus which generates, when an emergency braking is required, a brake force greater than that generated in a normal time is known. The above-mentioned conventional apparatus comprises a control circuit which generates a drive signal corresponding to an operational speed of a brake pedal and a fluid pressure generating mechanism which generates a brake fluid pressure corresponding to the drive signal generated by the control circuit.
The control circuit determines that, when an operational speed of a brake pedal is less then a predetermined value, the brake pedal is not normally operated. In this case, the fluid pressure generating mechanism is controlled so that a brake fluid pressure corresponding to a brake pressing force is generated. Hereinafter, this control is referred to as a normal control. Additionally, the control circuit determines that, when an operational force of the brake pedal exceeds a predetermined value, an emergency braking is required by the driver. In this case, the fluid pressure generating mechanism is controlled so that a brake fluid pressure is maximized. Hereinafter, this control is referred to as a brake assist control. Thus, according to the above-mentioned conventional apparatus, a brake force corresponding to a brake pressing force can be generated in a normal time, and a large brake force can be immediately generated in an emergency.
In the above-mentioned conventional apparatus, a normal braking operation and an operation requiring an emergency braking are discriminated in accordance with an operational speed of the brake pedal. Generally, the operational speed of the brake pedal when an emergency braking is required is higher than that of the normal braking operation. Thus, according to the above-mentioned discriminating method, the operation requiring an emergency braking and the operation requiring a normal brake can be discriminated with high accuracy.
When an amount of depression of the brake pedal is detected, it is determined that the emergency braking is not required any more and the control is switched from the brake assist control to the normal brake control. In this case, a fluid pressure generated by the fluid pressure generating mechanism is reduced discontinuously due to switching shocks. Thus, a vibration is generated in the fluid pressure of the brake force control apparatus, and such vibration is transmitted to the master cylinder or the brake pedal. If the vibration is transmitted to the brake pedal, an operational speed of the brake pedal may be increased and exceeds the above-mentioned predetermined value even though an amount of depression of the brake pedal is decreased. In this case, the brake assist control is unnecessarily started. Additionally, a noise may be superimposed on an output signal of the sensor which detects an operational speed of the brake pedal. As mentioned above, in the above-mentioned conventional apparatus, it is possible that an execution of the brake assist control is started immediately after the brake assist control is terminated even though an emergency braking is not required.